Process and system for separation and recovery of perfluorocompound gases

ABSTRACT

Processes and systems to recover at least one perfluorocompound gas from a gas mixture are provided. In one embodiment the inventive process comprises the steps of a) providing a gas mixture comprising at least one perfluorocompound gas and at least one carrier gas, the gas mixture being at a predetermined pressure; b) providing at least one glassy polymer membrane having a feed side and a permeate side; c) contacting the feed side of the at least one membrane with the gas mixture; d) withdrawing from the feed side of the membrane as a non-permeate stream at a pressure which is substantially equal to the predetermined pressure a concentrated gas mixture comprising essentially the at least one perfluorocompound gas; and e) withdrawing from the permeate side of the membrane as a permeate stream a depleted gas mixture comprising essentially the at least one carrier gas.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application No. 09/054,549, filedon Apr. 3, 1998, now U.S. Pat. No. 5,919,285, which is a continuation ofapplication No. 08/665,142, filed on Jun. 14, 1996 (now U.S. Pat. No.5,785,741), which is a continuation-in-part of application No.08/503,325, filed on Jul. 17, 1995.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to gas separation processes and more particularlythe separation and recovery (or disposal) of perfluorocompound gasesfrom a gas mixture. Especially, the invention relates to theconcentrating of low concentration gas mixtures of perfluorocompoundgases such as those present in the effluent of a semiconductormanufacturing process, particularly the etching and cleaning steps.

2. Related Art

The semiconductor industry is now using extensively perfluorocompoundssuch as CF₄, C₂F₆, C₃F₈, C₄F₁₀, CHF₃, SF₆, NF₃, and the like, in thesemiconductor manufacting processes involving gases, particularly in thevarious etching steps of the semiconductor manufacturing processes aswell as in the chamber cleaning step of the manufacturing process. Thoseperfluorocompound gases are used either pure or diluted, for examplewith air or nitrogen or other inert gas or in admixture with otherperfluorocompound gases or other carrier gases (for example inertgases). All of those gases do not necessarily react with other speciesduring the manufacturing processes: accordingly, when reactors arecleaned or evacuated to carry out another step of the manufacturingprocess, the effluent gases or gas mixtures should not be vented, evenif they are largely diluted with air or any other gas such as inert gas.Most of the perfluorocompounds (also called PFCs) have lifetimesmeasured in thousands of years in the atmosphere and are also stronginfrared absorbers. In the “Global Warming Symposium” held on Jun. 7-8,1994, in Dallas, Tex., USA, carbon tetrafluoride (CF₄), hexafluoroethane(C₂F₆), nitrogen trifluoride (NF₃), and sulfur hexafluoride (SF₆) havebeen identified as greenhouse gases of concern to the semiconductorindustry.

In the presentation made at this symposium by Michael T. Mocella andentitled “Perfluorocompound Emission Reduction From SemiconductorProcessing Tools: An Overview Of Options And Strategies”, the variouspossible strategies to control emission of these gases in the atmospherewere explained.

Apart from process replacement by non PFCs, several methods are alreadyknown or under development:

chemical-thermal decomposition with various activated metals wherein thespent bed material must be disposed. It is presently considered ascommercially unproven even if it is under promising development.

combustion-based decomposition process (or chemical-thermal process)using a flame to supply both the thermal energy and the reactants forthe decomposition. There are some safety issues associated with thehydrogen or natural gas fuels used and all the PFCs will produce HF as acombustion product (if the temperature is high enough), whose emissionsare also of concern and must be dealt with also. High temperatures mayalso be generated using a resistance heater.

plasma-based decomposition process which involves the use of a plasmasuch as coupled radio frequency systems to partially decompose C₂F₆,with over 90% decomposition of C₂F₆. However, such systems are not yetcommercially proven. Oxygen is usually needed to drive the decompositionto non PFC products with, however, the generation of HF which needs tobe thereafter managed.

recovery process wherein the PFCs are recovered instead of beingdestroyed in order to be recycled. This kind of process is of a greatinterest because it is considered as the “greenest” one. Differentschemes, according to the author, are possible “based on combinations ofadsorption or low temperature trapping of PFCs”. There are, however,several challenges such as dealing with the large amount of nitrogenassociated with the pump operation, the close boiling points of CF₄ andNF₃, the mixing of various process streams and/or possible reactionswith adsorbents. While recycle is suggested, there are obvious questionsabout recycling such mixtures.

Another combustion system for destroying high nitrogen content effluentgas streams comprising PFCs is disclosed in the article entitled “VectorTechnology's Phoenix Combustor” by Larry Anderson presented at the samesymposium June 7-8, 1994. This abatement system also uses a gas flame(using natural gas or hydrogen with air), which leads then to the sameproblem of HF generation and further destruction (plus the generation ofNO_(x), CO₂ inherent to any combustion process).

In the article presented at the same symposium by AT&T Microelectronicsand Novapure Corporation and entitled “PFC Concentration and Recycle”,the authors acknowledge the advantages of the recovery processes whichavoid production of carbon dioxide, NO_(x) and HF (compared tocombustion processes). Briefly, this process is disclosed as the use ofa dual bed adsorber (activated carbon), wherein one of the beds is inthe adsorption mode, while the second bed is regenerated: the PFCs areadsorbed on the carbon sieves while the “carrier” gases, such asnitrogen, hydrogen are not adsorbed and are vented to the exhaustsystem. When the system is switched on the second adsorber, then thefirst one is evacuated usmg a vacuum pump, then the effluent isrecompressed and the PFC gas mixture is recovered. One of the issues notyet resolved with such a system is that CF₄, which is non polar, is notreadily adsorbed by the carbon sieve and is then rejected with the ventgases. Also, any adsorption system is very sensitive to moisture and anytrace of water has to be removed from the feed flow.

It is also known from U.S. Pat. No. 5,281,255 to use membranes made ofrubbery polymers such as poly dimethyl siloxane or certain particularpolymers such as a substituted polyacethylene (having a low glasstransition temperature), to recover condensable organic componentshaving a boiling point higher than −50° C., essentially hydrocarbons(CH₄, C₂H₆, and the like), said hydrocarbons having the property ofpermeating through said membranes much faster than air, and thenrecovering on the permeate side of the membrane said hydrocarbons. Thepermeate (hydrocarbons) is then recovered at either substantiallyatmospheric pressure or lower pressure while the non-permeate (e.g. air)is still at the original pressure of the feed stream but is vented, andall of the pressure energy of the feed stream is lost.

Also, it is disclosed in U.S. Pat. No. 5,051,114 a selectivelypermeable. membrane formed from an amorphous polymer of perfluoro 2-2dimethyl 1-3-dioxole which is usable for separation of hydrocarbons orchlorofluorocarbons from, for example, air. Such a particular membraneapparently permeates oxygen and nitrogen faster than hydrocarbons andchlorofluorocarbons which can be recovered unexpectedly on thenon-permeate side of the membrane, contrary to all of the membranes,including those disclosed in U.S. Pat. No. 4,553,983 and 5,281,255. Inthe 114 patent, there is also disclosed a mixture of the amorphouspolymer of perfluoro 2-2 dimethyl 1-3 dioxole andpolytetrafluoroethylene. All these perfluoro polymers are known to beresistant to most of the harmful chlorofluorocarbons and hydrocarbonswhich make them commercially suitable for such separation. However, thismembrane is not currently available and there is no indication in thispatent whether or not such a membrane is suitable for separation of PFCsfrom air or nitrogen particularly at low concentrations of PFCs incarrier gases, and at widely varying feed flow conditions.

There is still presently a need for a “green” process for concentrationand/or recovery of PFCs from a gaseous stream, which can be used with afeed flow comprising or saturated with, moisture, which can handlesafely the PFCs recovery and/or concentration even with important orextreme variations of flows and/or concentration of PFCs in the feedstream, which does not produce hydrofluoric acid (HF) as a residue fromthe destruction of the PFCs (in addition to the possible HF content ofthe feed).

SUMMARY OF THE INVENTION

It has now been unexpectedly found that effluent gases, for example,from a semiconductor manufacturing process, which compriseperfluorocompounds can be treated efficiently by using certain,preferably hollow fiber, membranes which permeate much fister the“carrier gases” of the effluent gas mixture, such as air, nitrogen,oxygen, argon and/or helium, than the PFCs of the gas mixture which arethen recovered on the non-permeate side of the membrane.

Preferred membranes are glassy polymeric membranes, more preferablyasymmetric or composite (with an asymmetric outer layer) membranes.Preferably, these glassy polymeric membranes do not includeperfluorinated membranes. However, the glassy polymeric membranes usedin accordance with the invention can comprise a layer, including aposttreatment layer as disclosed in U.S. Ser. No. 08/138,309 filed Oct.21, 1993 (now abandond), and which is incorporated herein by reference,made of a fluorinated polymer such as polytetrafluoroethylene, amorphousperfluoro 2-2 dimethyl 1-3 dioxide, and the like.

One aspect of the invention relates to a process to recover at least oneperfluorocompound gas from a gas mixture, comprising the steps of:

a) providing a gas mixture comprising at least one perfluorocompound gasand at least one carrier gas, the gas mixture being at a predeterminedpressure;

b) providing at least one glassy polymer membrane having a feed side anda permeate side, the membrane being permeable to the at least onecarrier gas and being non-permeable to the at least oneperfluorocompound gaseous species;

c) contacting the feed side of the at least one membrane with the gasmixture;

d) withdrawing from the feed side of the membrane as a firstnon-permeate stream at a pressure which is substantially equal to thepredetermined pressure, a concentrated gas mixture comprisingessentially the at least one perfluorocompound gas, and

e) withdrawing from the permeate side of the at least one membrane as apermeate stream a depleted gas mixture consisting essentially of the atleast one carrier gas.

According to another aspect, the invention also relates to a process torecover a perfluorocompound gas or gas mixture from a gas mixtureflowing out from a semiconductor manufacturing process, comprising thesteps of pretreating the gas mixture to substantially remove most of theharmful components (gas, particles, and the like) to the membrane anddelivering a pretreated gas mixture, providing at least one glassypolymer membrane having a feed side and a permeate side, contacting thefeed side of the membrane with the pretreated gas mixture at a firstpressure, withdrawing the perfluorocompound gas or gas mixture from thefeed side of the membrane at a pressure which is substantially equal tothe first pressure and withdrawing a residue gas at a second pressurewhich is lower than the first pressure from the permeate side of themembrane. The semiconductor manufacturing process using PFCs may beselected from etching processes including oxide, metal and dielectric;deposition processes including silicon CVD, tungsten backetching, drychamber cleaning, and the like.

As some of the glassy membranes are sensitive to certain products whichmay be harmiful for them, i.e. which may destroy or plug them quickly,it is preferred to scrub the gas mixture prior to sending it on themembrane. Preferably any kind of species which is present in the feedflow stream which may harm the membrane is removed by the scrubbermeans, including any harmful gaseous HF, NH₃, WF₆, O₃, BCl₃, and anycorrosive species, also any pyrophoric species including siliconhydrides such as SiH₄, and any particulates having average diametergreater than about 20 micrometers, and any oil mists. Indeed, it ispreferred that compressors used in the methods and systems of theinvention be sealed and oil-free.

One preferred aspect of the invention relates to a process to recover atleast one perfluorocompound gas or gas mixture, comprising the steps of:

a) providing a glassy polymer membrane having a feed side and a permeateside;

b) providing a gas mixture at a first pressure comprising at least oneperfluorocompound gaseous species, at least one harmiful species for themembrane, and at least one carrier gas;

c) treating said gas mixture in scrubber means in order to substantiallyremove harmful species for said membrane and reduce the concentration ofsaid handful species to an acceptable level for said membrane andreceiving a scrubbed gas mixture at a second pressure;

d) contacting the feed side of said membrane with said scrubbed gasmixture at substantially said second pressure or at a higher pressure;

e) withdrawing a concentrated gas mixture comprising a higherconcentration of the at least one perfluorocompound gas than in thescrubbed gas mixture, from the feed side of the membrane as anon-permeate stream at a pressure which is substantially equal to saidsecond pressure; and

f) withdrawing a depleted gas or gas mixture from the permeate side ofsaid membrane as a permeate stream which is enriched in a carrier gasand depleted in the at least one perfluorocompound at a third pressure.

According to a preferred aspect of the invention, after concentratingthe PFCs with a membrane, the various PFCs are separated from eachother, by well known methods per se, such as selective condensation oradsorption in order to recover either separate PFCs or mixtures of PFCshaving close boiling points. According to another aspect of theinvention, the PFCs gas mixture is concentrated again, for example, witha second membrane, or the PFCs gas mixture is stored or recycled in theprocess (with or without additional treatment).

Other preferred process and system aspects of the invention includeprovision of a vacuum pump, heat exchanger, compressor, or cryogenicpump in order that the PFC gas mixture may be compressed, at leastpartially liquefied, and stored for future use. Another feature of theinvention includes the provision of a process step where the PFC gasmixture is concentrated using a plurality of membranes arranged inseries, with the possibility of the concentrated PFC gas mixture fromeach membrane unit being capable of use as a sweep gas of the permeateside of any one of or all of the membrane units in the series. A furtheraspect of the invention is the provision of a PFC gas mixture surge tankprior to the PFCs being recycled into the semiconductor manufacturingprocess, or prior to being routed to storage.

Another aspect of the invention is a semiconductor manufacturing systemcomprising:

a) at least one reactor chamber adapted to receive perfluorocompoundgases, carrier gases, and the like, the reactor chamber having a reactoreffluent gas conduit attached thereto;

b) at least one glassy polymer membrane separation unit having a feedside and a permeate side, the membrane being permeable to at least onecarrier gas and being substantially non-permeable to at least oneperfluorocompound gas, the membrane unit connected to the reactorchamber via the reactor effluent conduit, the membrane unit, having apermeate vent conduit and a non-permeate conduit, the latter adapted todirect at least a portion of a perfluorocompound containing non-permeatestream from the membrane unit to the reactor chamber. Preferred systemsin accordance with the invention include provision of pretreatmentand/or post-treatment means, such as dry or wet, (or both) scrubbers,thermal decomposers, catalytic decomposers, plasma gas decomposers andvarious filters as herein disclosed, prior to the reactor effluentstream entering the membrane unit. Also as herein disclosed, a pluralityof membrane units may be arranged in series, either with or withoutprovision of sweep gas of non-permeate on the permeate side of one orall membranes. Further preferred embodiments of systems of the inventionincluded a damper or surge tank in the non-permeate conduit (i.e.between the first or plurality of membrane units and the reactorchamber); and the provision of a compressor, heat exchanger, cryogenicpump or vacuum pump on one or more of the non-permeate, PFC enrichedstream(s), allowing the PFC enriched stream(s) to be stored in liquidform for future use. Also preferred are appropriate valves which allowthe damper or surge tank, and the compressor for creating the liquid PFCmixture, to be bypassed, as explained more fully herein.

Preferred processes and systems of the invention include operating oneor more of the membrane units at a constant concentration set-point forthe PFC concentration in the non-permeate stream from each membraneunit. In this preferred system and process, the set-point concentrationof the PFC in the non-permeate stream from each succeeding PFC membraneseparation unit would of course be higher than the immediately precedingone. Appropriate sensors can be inserted into the non-permeate effluentconduit from each membrane unit to continuously or non-continuouslyanalyze for PFC concentration, or, samples may be taken periodically orcontinuously from the non-permeate effluent from each membrane unit,which may be sent to dedicated analyzers either on-site or off-site.This information is preferably then forwarded to a process controllerwhich may control for example the pressure of the feed to each membraneunit, temperature, flow, and the like. Also, when discussing the use ofa sweep gas arrangement, the sweep gas may either be controlled via anopen loop or a closed loop arrangement.

Another preferred system and process embodiment of the present inventionincludes the recycle of the permeate stream of either the first orsucceeding stages of the membrane units (in other words, the carrier gasand other process gases are recycled). The carrier gases may be recycleddirectly to the reactor chambers, or may be delivered to heatexchangers, compressors, and the like to reduce them to liquid form forstorage or future use. A recycle membrane may be provided, functioningto separate carrier gases from process gases.

Other preferred processes and systems of the invention are those whereina waste stream from a pretreatment step for the gas mixture emanatingfrom the semiconductor process is used to generate one or moreperfluorocompounds or other chemicals, which may then be purified foruse in the semiconductor process, or other chemical processes, as morespecifically described in assignee's copending application Ser. No.08/666,694 filed Jun. 14, 1996, which is incorporated herein byreference.

Still other preferred processes and systems in accordance with theinvention are those wherein one or more non-permeate streams ispost-treated to remove non-perfluorocompounds. Post-treatment methodsinclude those previously mentioned as suitable for pretreatment of thefeed gas to the membrane.

Another aspect of the invention is a method of recovery of a relativelypure PFC stream from a vent stream from one or more gas cabinets, tubetrailers, clean rooms, or the like using a membrane unit as describedherein.

Further understanding of the processes and systems of the invention willbe understood with reference to the brief description of the drawing anddetailed description which follows herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph illustrating the efficacy of destruction of PFCs witha burner versus the burner flame temperature (prior art);

FIG. 2 represents a schematic drawing of one process and systemaccording to the invention;

FIG. 3 is a detailed view of a portion of the process and system of FIG.2;

FIGS. 4-7 illustrate four different embodiments of the invention;

FIG. 8 illustrates PFC concentration on the residue side (permeate) ofthe membrane versus the pressure differential across the membrane, fordifferent flowrates of the feed stream;

FIG. 9 illustrates PFC concentration on the residue side (permeate) ofthe membrane versus the pressure differential across the membrane, fordifferent temperatures of the feed flow,

FIG. 10 illustrates PFC concentration on the recovery side (non-permeateside) of the membrane versus the pressure differential across themembrane, for different flowrates of the feed stream;

FIG. 11 illustrates PFC concentration on the recovery side (non-permeateside) of the membrane versus the pressure differential across themembrane, for different temperatures of the feed flow;

FIG. 12 illustrates schematically a prior art gas cabinet;

FIG. 13 illustrates schematically a gas cabinet including a membranerecovery unit; and

FIG. 14 illustrates multiple gas cabinets venting into a common membranerecovery unit.

DESCRIPTION OF PREFERRED EMBODIMENTS

Recovery of PFCs, for example, from a semiconductor manufacturingprocess, is now made possible by the present invention using certaintypes of polymer membranes and concentrating a gas mixture comprisingPFCs by recovering the non-permeate flow on the non-permeate side of themembrane, while non harmful gases for the environment permeate throughthe membrane and can then be directly vented or recycled. This processis simpler and environmentally friendlier than many existing processes,as described hereabove. The non-permeate stream may either be reroutedto the semiconductor manufacturing reaction chamber, routed to a storagefacility for future use, or routed to a PFC recovery apparatus forseparation of individual or like PFCs, either on-site or off-site priorto reuse.

Perfluorocompounds, for the purpose of this invention, are defined ascompounds comprising C, S and/or N atoms wherein all or all but onehydrogen have been replaced by fluorine. The most common PFCs include,without being limited to, any of the following compounds: fullyfluorinated hydrocarbons such as CF₄, C₂F₆, C₃F₈, C₄F₁₀, and otherfluorinated compounds such as CHF₃, SF₆, NF₃, and which are not harmfulfor the membrane. In certain cases, PFCs may also include BF₃, COF₂, F₂,HF, SiF₄, WF₆, WOF₄, as long as they are not harmful for certain typesof membranes. Perfluorocompounds do not include chlorofluorocarbons, orcompounds comprising two hydrogen substituents or more, since suchcompounds do not usually behave as PFCs vis a vis the membrane and arenot useful in semiconductor manufacturing processes.

Membranes useful in the invention are preferably glassy membranes, suchas polymer membranes made preferably from polyimides, polyamides,polyamide-imides, polyesters, polycarbonates, polysulfones,polyethersulfone, polyetherketone, alkyl substituted aromaticpolyesters, blends of polyethersulfone, aromatic polyimides, aromaticpolyamides, polyamides-imides, fluorinated aromatic polyimide, polyamideand polyamide-imides, glassy polymeric membranes such as disclosed inU.S. Ser. No. 08/247,125 filed May 20, 1994 now U.S. Pat. No. 5,599,380and incorporated herein by reference, cellulose acetates, and blendsthereof copolymers thereof substituted polymers (e.g. alkyl aryl)thereof and the like.

Asymmetric membranes are prepared by the precipitation of polymersolutions in solvent-miscible nonsolvents. Such membranes are typifiedby a dense separating layer supported on an anisotropic substrate of agraded porosity and are generally prepared in one step. Examples of suchmembranes and their methods of manufacture are disclosed in U.S. Pat.Nos. 4,113,628; 4,378,324; 4,460,526; 4,474,662; 4,485,056; 4,512,893,5,085,676, and 4,717,394 all incorporated herein by reference. The '394and '676 patents disclose preparation of asymmetric separation membranesfrom selected polyimides. Particularly preferred membranes are polyimideasymmetric gas separation membranes as disclosed in the '676 patent.

In a pressure driven gas membrane separation process, one side of thegas separation membrane is contacted with a complex multicomponent gasmixture and certain of the gases of the mixture permeate through themembrane faster than the other gases. Gas separation membranes therebyallow some gases to permeate through them while serving as a barrier toother gases in a relative sense. The relative gas permeation ratethrough the membrane is a property of the membrane material compositionand its morphology. It has been suggested in the prior art that theintrinsic permeability of a polymer membrane is a combination of gasdiffusion through the membrane, controlled in part by the packing andmolecular free volume of the material, and gas solubility within thematerial. Selectivity is the ratio of the permeability's of two gasesbeing separated by a material. It is also highly desirable to formdefect-free dense separating layers in order to retain high gasselectivity.

Composite gas separation membranes typically have a dense separatinglayer on a preformed microporous substrate. The separating layer and thesubstrate are usually different in composition. Composite gas separationmembranes have evolved to a structure of an ultrathin, dense separatinglayer supported on an anisotropic, microporous substrate. Thesecomposite membrane structures can be prepared by laminating a preformedultrathin dense separating layer on top of a preformed anisotropicsupport membrane. Examples of such membranes and their methods ofmanufacture are disclosed in U.S. Pat. Nos. 4,664,669; 4,689,267;4,741,829; 2,947,687; 2,953,502; 3,616,607; 4,714,481; 4,602,922;2,970,106; 2,960,462; 4,713,292, 4,086,310; 4,132,824; 4,192,824;4,155,793; and 4,156,597, all incorporated herein by reference.

Alternatively, composite gas separation membranes may be prepared bymultistep fabrication processes, wherein first an anisotropic, poroussubstrate is formed, followed by contacting the substrate with amembrane-forming solution. Examples of such methods are described inU.S. Pat. Nos. 4,826,599; 3,648,845; and 3,508,994, all incorporatedherein by reference.

U.S. 4,756,932 describes how composite hollow-fiber membranes may alsobe prepared by co-extrusion of multiple polymer solution layers,followed by precipitation in a solvent-miscible nonsolvent.

According to one embodiment of the present invention, the membrane canbe post-treated with, or coated by, or coextruded with, a fluorinated orperfluorinated polymer layer in order to increase its ability towithstand harmful constituents in the gas mixture from which PFCs are tobe separated, at low levels or temporary contact with such components.

The hollow-fiber spinning process depends on many variables which mayaffect the morphology and properties of the hollow-fiber membrane. Thesevariables include the composition of the polymer solution employed toform the fiber, the composition of fluid injected into the bore of thehollow-fiber extrudate during spinning, the temperature of thespinneret, the coagulation medium employed to treat the hollow-fiberextrudate, the temperature of the coagulation medium, the rapidity ofcoagulation of the polymer, the rate of extrusion of the fiber, takeupspeed of the fiber onto the takeup roll, and the like.

The gas mixture containing PFCs to be separated usually comprises atleast one PFC and at least one carrier gas such as air, nitrogen, argon,helium, or the like and mixtures thereof

In Table I are listed the most usual PFCs and other gases from wastestreams from a semiconductor manufacturing process (not all ofthose-gases are necessarily present—only some of them may be present inthe exhaust).

The most common PFCs are usually the following ones:

for chamber cleaning: carbon tetrafluoride (CF₄), hexafluoroethane(C₂F₆), nitrogen trifluoride (NF₃), perfluoropropane (C₃F₈), sulfurhexafluoride (SF₆), trifluoromethane (CHF₃);

for the etching steps, the same PFCs are usually used but with severalother gases such as argon, boron trichloride, chlorine, hydrogenbromide, hydrogen chloride, hydrogen fluoride, phosphine, silane,silicon tetrachloride, and the like.

Some of these gases are sometimes harmful for the membrane (as indicatedin Table I), and it is preferred to remove them or destroy them from thefeed gas mixture sent to the membrane. Usually it is preferred to removethe following compounds prior to sending the flow to the membrane: WF₆,HF, F₂, NH₃, Cl₂, HBr, HCl, O₃, and any silicon hydrides, germaniumhydrides, and the like. To do this, various methods can be used such asusing scrubber means (dry or wet scrubbers), thermal decomposition,plasma destruction, catalytic removal and the like, to reach a levelusually below about 1% vol. of said harnfil substance in the feed.However, it is preferred to reach a level for each harmful substancelower than 10 ppm, most preferably lower than 1 ppm. It is also possibleto treat the separated PFC non-permeate stream using one or more ofthose methods, referred to herein as post-treatment.

TABLE I Symbol Name Harmful to membrane PFCs C₂F₆ Hexafluoroethane notharmful CF₄ Tetrafluoromethane not harmful CHF₃ Trifluoromethane notharmful NF₃ Nitrogen trifluoride not harmful SF₆ Sulfur hexafluoride notharmful C₃F₈ Perfluoropropane not harmful COF₂ Carbonyl fluoride notharmful Other gases (carrier gases, etc.) Ar Argon not harmful AsH₃Arsine not harmful BCl₃ Boron trichloride not harmful BF₃ Borontrifluoride not harmful CH₃OH Methanol not harmful Cl2 Chlorine harmfulabove 1% F₂ Fluorine harmful above 1% H₂ Hydrogen not harmful HBrHydrogen bromide harmful above 1% HCl Hydrogen chloride harmful above 1%HF Hydrogen fluoride harmful above 1% He Helium not harmful N₂ Nitrogennot harmful N₂O Nitrous oxide not harmful NH₃ Ammonia harmful above 1%NO Nitric oxide not harmful O₂ Oxygen not harmful O₃ Ozone harmful above1% Si(OC₂H₅)₄ Tetraethyl Orthosilicate (TEOS) not harmful PH₃ Phosphinenot harmful SiF₄ Silicon tetrafluoride not harmful SiH₄ Silane harmfulabove 1% WF₆ Tungsten hexafluoride harmful above 1% WOF₄ Tungstentetrafluoride oxide not harmful

SiF₄, WF₆, WOF₄, HF, F₂ while being perfluorinated compounds are usuallynot considered as PFCs.

The scrubber means to remove the harmful product for the membrane can bea dry scrubber (which usually removes at least F₂, HF, HCl, HBr, Cl₂,NH₃, WF₆ and SiH₄). Dry scrubbers are usually resin-type scrubbers, orsoda-lime, while some dry scrubbers comprising catalysts like MnO₂ canalso remove ozone. Also, gaseous hydrides may be removed according tothe methods disclosed in U.S. Pat. Nos. 4,743,435; 4,784,837; 4,910,001;4,996,030, 5,182,088 and 5,378,439 incorporated herein by reference.When different scrubbers have to be installed in order to remove thevarious harmful constituents, it is preferred to install first the dryscrubber (or scrubbers) and then the wet scrubber. Upstream before thescrubbers, filters to remove particles from the stream are usuallynecessary (removal of particles having a diameter larger than 20microns) and it is recommended according to the invention to provide afilter upstream having a pore size diameter less than 20 micrometers andpreferably less than 10 micrometers, which removes particles and liquiddroplets to avoid plugging of the membrane.

A wet scrubber is, for example, disclosed in the brochure entitled“Selecting a CDO™ for your Particular Application” from DELATECHCorporation, which brochure is incorporated herein by reference.

According to a preferred aspect of the invention, there exist somerelationship between the pressure drop across the membrane (i.e. ΔPbetween the feed and the permeate), the temperature of the feed (i.e.the temperature of the membrane after temperature equilibration betweenthe feed flow and the membrane itself) and the feed flowrate. It hasbeen discovered that, for a certain constant flowrate of the feed gas onthe membrane and temperature of the feed gas, when the pressuredifferential across the membrane increases, the recovery of PFCs likee.g. C₂F₆ decreases on the non-permeate or “residue” side of themembrane while this PFCs concentration increases on the permeate side ofthe membrane. Accordingly, it is preferred, according to the invention,to have a pressure drop ΔP across the membrane which is not high,usually smaller than about 13,600 kPa (2000 psig), preferably rangingfrom about 120 to about 1450 kPa (from about 3 to about 200 psig) andmost preferably from about 240 and to about 510 kPa (from about 20 andto about 60 psig).

As far as the feed gas mixture is usually obtained at substantiallyatmospheric pressure, there is either the option to compress this feedto have a sufficient pressure drop across the membrane (but this is notpreferred because usually, many of the species present in the feed maydeteriorate the compressor) or create on the permeate side of themembrane a pressure lower than atmospheric pressure (which may bepreferred because most of the species which may harm the vacuum meansare retained on the non-permeate side of the membrane). To create thislowered pressure on the permeate side of the membrane, a vacuum pump orany other suction means is usually adequate. Alternatively, if the feedstream to the membrane is to be compressed, compression is preferablycarried out after the feed stream has been pretreated using wet or dryscrubbers, filters, catalytic removal, pulsed corona destruction,thermal decomposition, and/or plasma decomposition, as explained incopending application Ser. No. 08/663,884 file Jun. 14, 1996incorporated herein by reference. Preferred compressors are sealed andoil-free, such as the compressors known under the trade designationPOWEREX available from the Powerex Harrison Company, of Ohio, USACompression ratio (defined as the ratio of pressure at the compressoroutlet divided by the pressure at the compressor inlet) of thecompressor which feeds the membrane unit (or the first membrane unit ofa series of membrane units) generally ranges from about 2:1 to about10:1, it being appreciated that supplemental compression may be requiredat other membrane feed locations in a series of membrane units. It maybe necessary to provide heat exchange between the compressed feed streamand a coolant, such as liquid nitrogen, for example if the temperatureand/or pressure of the feed flowing into a particular membrane is to becontrolled, or the PFC concentration in the non-permeate stream iscontrolled at a set-point value, as disclosed herein.

Whatever pressure drop across the membrane is chosen, according to thedisclosure given hereabove, it is preferred to have a higher feed flowthan a lower feed flow (even if such a system can work with a variableflowrate of the feed): the highest the feed flow, the highest therecovery. This feed flow can vary from near zero to about 10⁵ Nm³/h persquare meter of membrane available for separation, preferably from about10⁻⁴ to about 10 Nm³/h-m² and more preferably from about 0.1 to about0.5 Nm³/h-m².

Alternatively, the compressor may be positioned after the pretreatmentmeans (dry and/or wet scrubbers, filters, and the like).

The temperature of the feed flow and/or the membrane shall also have aninfluence on the recovery of PFCs on the non-permeate side of themembrane. Usually, when the feed and/or the membrane temperatureincreases, then for a constant flowrate and pressure drop, the speciesof the gas mixture tend to permeate more through the membrane,particularly those which already permeate faster at lower temperature.For example, nitrogen and oxygen (air) which permeate much fasterthrough the membrane than the PFCs at ambient temperature will permeateeven much faster through the membrane at higher temperature, e.g. 50° C.to 60° C.

Usually, the temperature of the feed and/or the membrane can vary fromabout −10° C. to about 100° C., preferably from about 10° C. to about80° C., and particularly preferably ranging from ambient temperature(usually about 20° C. to 25° C.) to about 60° C.

Another preferred method of operating the membrane separation units ofthe process and system of the invention is by operating each membraneunit to have a constant, set-point concentration of one or more PFCgases in the non-permeate stream exiting one or more of the membraneunits. Some of the advantages of operating the process and system of theinvention in this manner are that feed pressure fluctuations can besmoothed, and that the life of the membrane can be prolongedsignificantly. One way of maintaining the set-point concentration of thePFC in the non-permeate stream is to pass a portion of the non-permeatestream, preferably countercurrently, by the external side of the hollowfiber membrane (that is, on the permeate side of the hollow fibers ofthe membrane unit). This practice is more fully described in U.S. Pat.Nos. 5,240,471 and 5,383,957, both assigned to L'Air Liquide S.A withthe exception that the patents do not describe separation of PFCs usingthese techniques. Both of these patents are incorporated herein byreference for their teaching of sweep gas techniques. Thus, a portion orall of a non-permeate stream of stage N can be used as feed stream forstage N+1 and/or N+2, etc., bearing in mind that there is usually, inpractice, a small pressure drop between stage N, stage N+1 and stageN+2, etc. This means that the pressure on the non-permeate (feed side)of stage N is greater than the pressure on the feed side of anysubsequent stage, such as N+1 or stage N+2.

After this first concentration step with one or a plurality ofmembranes, it is preferred to then carry out a second step wherein thevarious PFCs are at least partially separated from each other, or moreabundant PFCs separated from minor amounts of other PFCs. Differentseparation techniques for separating two or more perfluorocompounds canbe used such as distillation, adsorption, condensation, and the like.Preferably, and because it may be more appropriate for the streams whichare coming out of a semiconductor manufacturing tool a condensationprocess can be used such as the one known under the tradename SOLVAL ofAir Liquide America Corporation disclosed in the Technical Bulletinentitled “Solval™ Solvent Condensation and Recovery System”, 1994, andincorporated herein by reference. Basically, in this condensationprocess, the effluent from the non-permeate side of one or a pluralityof membranes is fed into a heat exchanger. Liquid nitrogen or anothercooling medium is introduced into the heat exchanger and flows throughthe cooling coils. The mixture of PFC with N₂ is introduced into theshell of the heat exchanger and flows around the coils as it passesthrough the shell. The mixture is cooled and part of the PFC vapors arecoalesced, liquefied and collected based upon the temperature at thecooling coils. The higher the liquid nitrogen flowrate into theexchanger, the lower the temperature at the cooling coils, andtherefore, more of PFCs will be liquefied.

In some preferred embodiments, the PFC mixture after concentrationcomprises species whose boiling points are close and it is thereforedifficult to separate them by fractional condensation. For example, C₂F₆has a normal boiling point of −78.2° C. and CHF₃ has a normal boilingpoint of −82.1° C.; CF₄ has a normal boiling point of −127.9° C. and NF₃has a normal boiling point of −129° C. When it is desired to separate amixture comprising at least two species having close boiling points,then a first separation by, for example, condensation is made betweenthe various species having boiling points not too close from each otherin order to provide substantially pure species or a mixture of specieshaving close boiling points. Then, the mixture of species having closeboiling points are separated by another process, for example, adsorptionwhen one of the species of the mixture is more polar than the other. NF₃and CF₄ may be separated using molecular seives (such as NaX CaX, andNaA, wherein the “A” designates 5 Angstrom cage size, and the “X”designates a 10 Angstrom cage size); activated carbon; or the like,wherein the polar species (such as NF₃ and CHF₃) are preferentiallyadsorbed, as opposed to non-polar species such as CF₄.

FIG. 1 illustrates the efficacy of a burner to destroy PFCs versustemperature (° C.) in a prior art process. For example, when an air-fuelburner is used, the temperature of the flame which is reached, if almost100% of NF₃, CCl₂F₂ (which is not a PFC but is chlorofluorocompound usedby the electronic industry), CHF₃ and SF₆ are destroyed (generating HFand other undesirable species), C₂F₆ and CF₄ are only partiallydestroyed, particularly C₂F₆ which is only 50% destroyed: the combustiongases cannot accordingly be vented. However, when using an oxy-fuelflame which temperature is about 1400° C., it is possible to destroymost of the C₂F₆, while still generating undesirable species. In thepresent invention, combustion at 900° C. may remove all PFCs but C₂F₆and CF₄, which can then be recycled together.

The general features of one process according to the invention areillustrated in FIG. 2, wherein a semiconductor manufacturing process isrepresented by the reference numeral 1 (which may be any type of processusing PFCs and rejecting PFCs). The PFCs and carrier gases fed toprocess 1 are represented by 23 and 22, respectively (bulk and/orcylinder delivery through traditional bulk systems or gas cabinets wellknown in the electronic industry).

A waste gas mixture of PFCs, carrier gases and any other gases 24 (suchas chemically reactive gases) is recovered from process 1 in an exhaustline 2. The waste gas mixture is preferably passed through filter 5 a,and then compressed in a compressor C. The compressed gas mixture isthen optionally routed to a cooler or heater Q to provide a desiredtemperature for the compressed gas mixture. The gas mixture is thenpreferably scrubbed in a dry scrubber 3 to remove most of siliconhydrides, NH₃, AsH₃, tetraethylorthosilicate (TEOS), halogen, halides,then preferably scrubbed in a wet scrubber 4 to remove most of hydrides,halides, halogen gases (according to the nature of the gas mixtureprovided in 2, only dry scrubber 3 or wet scrubber 4 may be necessary),then filtered in a filter 5 b to remove dust, particles, droplets, andthe like, having size greater than 20 micrometers. Additionally,particles and dust may be removed in a filter upstream from dry scrubber3. A gas mixture in 25 no longer contains any substantial amount ofharnful component for a membrane unit 6. Gas stream 25 is sent on thefeed side (bore side) of a plurality of hollow fibers of membrane unit6, the carrier gases of the nature then permeate through the hollowfibers of membrane unit 6 and are recovered or vented as a waste gas 7(if for example, the carrier gas comprises helium, and also argon, itmay be useful to recover it and recycle it in the process, with furtherpurification or not). The non-permeate stream which comprises the PFCs(concentrated) are recovered in 8 and either directly recycled toprocess 1 (or stored in bulk to be later reused in process 1) through aline 9 or sent to a separation unit, for example a condensation unit 10.In condensation unit 10, a heat exchanger receives liquid nitrogen LN₂in line 15 and discards typically a mixture of LN₂/GN₂ in line 16, wichcondenses the high boiling point species (by using different flowratesof LN₂, one can easily control the condensation of various products)which are recovered as a liquid on line 12 and sent to, for example, toan adsorption process 11 which separates the polar fraction from thenon-polar fraction (respectively 19, 20), which fractions are eitherrecovered in 21 for further treatment on-site or off-site (the dottedlines indicate that this is not the preferred alternative) orrecycled/stored in process 1.

The gaseous fraction is sent through line 14, for example a pressureswing adsorption system 13 (or any other adsorption system) wherein theadsorbed species (oneor several) are recovered on line 17 and whereinthe non-adsorbed species (one or several) are recovered on line 18. Bothproducts on lines 17 and 18 are either recovered in 21 (for exampleoff-site treatment) or recycled in process 1.

Those species or mixture of species are either recycled in process 1 orrecovered in the PFC recovery unit 21. At 24 are the other gas inlets inthe process (for example chemical gases other than PFCs and other thancarrier gases used to dilute the other gases or to purge a chamber).Those other gases are sometimes those which are harmful for the membrane(for example SiH₄, WF₆, and the like) and which are used in other stepsof the manufacturing process of a semiconductor.

FIG. 3 is a detailed partial view of FIG. 2 of the membrane system andthe condensation system. A feed stream 41 (wherein all harmfulcomponents have been removed) is compressed in compressor 40 and thestream is fed to the feed side 43 of membrane 42. The permeate stream 45from the permeate side 44 of the membrane is usually vented. A pressureregulator 46 which may or may not be required controls the pressuredownstream the membrane (on the non-permeate stream), while thenon-permeate stream 47 is fed, for example, to a condensation system 48,which separates by heat exchange with liquid nitrogen LN₂ the condensedstream or liquid stream 49 from the uncondensed stream or gaseous stream50. After heat exchange, the liquid nitrogen LN₂ is substantiallytotally vaporized as gaseous nitrogen GN₂.

FIG. 4 represents a simplified schematic diagram of one process andsystem embodiment of the invention. Feed gas 90 from a semiconductormanufacturing process is compressed in a compressor 92 prior to enteringa first stage membrane M1. First stage membrane M1 creates a permeatestream 94 comprised primarily of carrier and process gases, and anon-permeate stream 96, enriched in one or more PFCs. A back pressureregulator 97 provides a pressure drop across membrane M1. Non-permeatestream 96 then enters a second stage membrane M2, producing a secondnon-permeate PFC enriched stream 98, and a second stage permeate stream100 comprised primarily of carrier and process gases which areimpermeable to the M1 membrane but which are permeable to the M2membrane. A second back pressure regulator 99 maintains a pressure dropacross second stage membrane M2. Optionally, streams 94 and 100 may becombined and either disposed of, or recycled as shown for stream 100.Optional and preferred components of the system embodied in FIG. 4include provision of a valve 104 and conduit 106 which allow a portionof the PFC product stream 98 to be swept across the permeate side ofmembrane M2, thereby affording process efficiencies as described in the'471 and '957 patents, previously incorporated by reference Also, anoptional vacuum pump is illustrated at 102 on the recycled gas stream.Vacuum pump 102, if present, allows recycled gas stream 100 to reenterthe system with the feed gas.

FIG. 5 illustrates a system and process substantially in accordance withthat of FIG. 4, with the provision of a recycle membrane M_(R) inrecycle gas line 100. Also provided is a conduit 110 and vacuum pump 112which allows separation via recycle membrane M_(R) of carrier gases.Thus the recycle gases in conduit 108 are comprised primarily of otherprocess gases as defined herein.

FIGS. 6 and 7 illustrate two other possible embodiments of theinvention. In FIG. 6, several identical or different processes 60 . . .61 are available for use (either simultaneously or not), using similaror different PFC gases and other gases designated as process gases. Thegas exhausts from 60 . . . 61 are preferably scrubbed in scrubbers S₁,S_(N), and then are preferably diluted with N₂, and compressedrespectively in 62 . . . 63, and mixed together as a single stream 67(in fact the various processes 1 . . . N may either successively orsimultaneously discharge exhaust gases). Single stream 67 is thenpreferably filtered at S_(m) as a final cleaning step, then fed to amembrane unit M1 of the invention wherein the permeate 65 may be ventedor recycled in line 65 a to a recycle membrane which separates usableprocess gases from carrier gases. A non-permeate 66 and 66 a(concentrated PFCs) may be recycled to one or several of the processes 1. . . N, respectively 60 . . . 61. Alternatively, non-permeate stream 66c may be routed to one or more membrane units M₂, M₃, . . . M_(N), thusimproving the purity of the PFCS. Preferably, a sweep gas stream 66 bmay be employed to sweep the permeate side of M1. Membrane units M₂, M₃and M_(N) also may have sweep gas streams. Other preferred features ofthe invention include the provision of one or more vacuum pumps (69 aand 69 b), a high pressure PFC storage vessel 68, and/or a surge tank64. As an example of multiple different processes, one process may be ametal oxide etch, another might be an oxide etch, and yet another mightbe a tungsten CVD process.

Further illustrated in FIG. 6 is a mass flow measurement device 100 onthe non-permeate stream 66 of membrane unit M₁ which may be used tocontrol the flow of the permeate stream indirectly via controller 200,which accepts a signal from flow measurement device 100 and adjusts flowcontrol valves in conduits 67 and 65. Conduit 66 also preferablyincludes a backpressure regulator, which is not illustrated for clarity.Backpressure regulators 102, 104, and 106 serve the function asdescribed above for backpressure regulator 97 (FIG. 4). Other processcontrol schemes are certainly feasible. For example, it may beadvantageous, as previously mentioned, to operate Ml using a set pointPFC concentration in the non-permeate stream 66. In such embodiments,the flow measurement device may also include analysis equipment todetermine the PFC concentration in conduit 66. Similar process controlsmay be used for membrane units M1, M2, M3, and MN as desired. Also,similar piping arrangements may be employed in the latter membraneunits, as denoted at 108 a, 108 b, 108 c, 104 a, 104 b, 104 c, 106 a,106 b, and 106 c.

FIG. 7 is a parallel processing embodiment wherein each process 1 . . .N is associated (after dilution with nitrogen and compressionrespectively in 72 and 73) with a membrane system M₁ . . . M_(N),respectively, according to the invention. Each feed stream 74 . . . 75is fed to a membrane system M₁ . . . M_(N) (with pretreatment systems S₁. . . S_(N) if necessary). The permeate gases are vented together at 78while each non-permeate 79, 80 is recycled, preferably to itscorresponding process. Alternatively, rather than venting all of thepermeate streams through vent line 78, permeate from one or moremembranes, for example, membrane M₁, may be recycled through variousconduits 76, 76 a, and 78 a, as illustrated in FIG. 7. Furthermore,non-permeate streams 79, 80, and 81 may be recycled to theircorresponding process through lines 79 a, 80 a, and 81 a, respectively.Some non-permeate streams may be used as sweep gases on the permeateside of each membrane. The sweep gases are preferably provided throughlines 80 b, 81 b, and 79 b, for membranes M₁, M_(s), and M_(n),respectively. Preferred systems of the invention include a redundantmembrane unit M_(s), preferably having its own pretreatment unit S_(T).Also preferred is a recycle membrane unit M_(R), which separates usablereactive process gases from carrier gases. Note that with suitablearrangement of valves, this embodiment can operate in parallel or series(cascade) mode.

It is important to note that for all these different embodiments of theinvention, it may in some instances be preferred to create a pressuredrop across the membrane. In one embodiment this may be done by creatingvacuum on the permeate side of the membrane while keeping the feed gasat about atmospheric pressure, which is usually about the pressure ofthe gas mixture released from the semiconductor manufacturing process.As long as usually only the carrier gases permeate through the membrane,those gases cannot damage a vacuum pump or other vacuum system, while onthe contrary, compressing the gas mixture upstream from the membranewould not only mean compressing more gas, but it would also mean a riskfor the compressor means.

FIG. 8 illustrates at 20° C. for two different flowrates of the feedflow of 170 ml/min and 130 ml/min, respectively, on a hollow-fibermembrane made of polyimide having a surface of about 0.2 m² wherein thefeed flow is sent into the hollow fiber with a permeation towards theoutside hollow fiber. FIG. 8 clearly illustrates for low pressure dropbetween the non-permeate and the permeate sides of the membrane, noconcentration of C₂F₆ occurs (0.2% of C₂F₆ recovered on the non-permeateside with the “residue”). For higher pressure drops, depending on thefeed flow, the concentration of C₂F₆ then increases with an onset pointof about 7×10⁵ N/m² (ΔP across the membrane) for a feed flow of 130ml/min. For higher flowrates (170 ml/min.) the onset point is obviouslyhigher (increases with feed flow).

FIG. 9 illustrates the effect of the temperature of the feed flow (or ofthe membrane)—same membrane as used for FIG. 8. For a higher temperatureof the flow, a higher differential pressure across the membrane isneeded to achieve the same concentration of PFCs.

FIG. 10 illustrates the recovery rate of C₂F₆ on the non-permeate sideof the membrane versus the differential pressure across the membrane fortwo different flowrates: for very low differential pressure, about allof the C₂F₆ is recovered while the rate of C₂F₆ permeating through themembrane progressively increases with the pressure drop across themembrane, such rate increasing faster for lower flowrates (comparecurves for 130 ml/min. and 170 ml/min).

FIG. 11 illustrates the effect of temperature for a flow of 170 ml/min.:while only an extremely low amount of C₂F₆ permeates at 20° C., almosthalf of it permeates at 55° C. for a pressure drop of about 7×10⁵ Nlm².

From a recovery standpoint (FIGS. 10 and 11), it is thus better tooperate at high flowrates and ambient temperature for a given pressuredrop. But FIGS. 8 and 9 indicate that a substantial pressure drop isnecessary to have a certain purity of C₂F₆ (and thus a certainconcentration).

FIGS. 12-14 illustrate another aspect of the invention. Gas cabinets(sometimes known as gas panels) are well known in the semiconductormanufacturing art and need little explanation to the skill artisan. Agas cabinet for PFCs will have a PFC vent stream. PFC vents from tubetrailers, clean rooms, gas packaging facilities, and the like, also needlittle explanation. Although the following discussion is for gas cabinetvents, the idea pertains to the recovery of a relatively pure PFC streamfrom any venting of PFCs.

All automated cabinets employ specific purging routines before and aftercylinder change. The pre-purge routine generally is utilized to purgeprocess gas while a post-purge routine is generally used to removepurging intrusions. The following describes different operational modesof a typical gas cabinet. see FIG. 12.

Process Gas Delivery Mode

Process gas flows first through V1, then through a pressure reducingregulator and then through V7. Valves V2, V3 and V5 are bypassed duringthe process gas delivery mode.

Pre-purge Routine

Vent Purge

Once the process gas cylinder valve has been closed, the vacuumgenerator is activated with nitrogen by V4 and the vent line is purgedfor atmospheric removal. The vent area between V2 and V5 is alsoevacuated at this time. V4 is then cycled on and off to back fill thearea between V2 and V5 with nitrogen.

Vent Mode

With V7 closed and V4 on, the process gas is vented through V5 and V1remains open until the process gas safely reaches atmospheric pressure(sensed by a pressure transducer). A 0.040 inch orifice is located onthe vacuum inlet of the vacuum generator to restrict the venting flowrate.

Vacuum Mode

With V4 on, V1 closed and V2 opened, a vacuum of 22-24 inches Hg isgenerated between V1 and the cylinder valve. Note: V1 is always closedduring the purging routine.

Nitrogen Pressure Mode

With V4 on, V2 and V3 opened, V6 is opened allowing nitrogen pressure(80 psi) to overcome the vacuum and to pressurize the system withnitrogen between V1 and the cylinder valve. When V6 is closed, a vacuumis then again generated between V1 and the cylinder valve. V6 is cycledon and off multiple times in the pre-purge to create the vacuum/pressurepurging action.

Nitrogen Purge Gas Bleed

Once pre-purge is complete, a pigtail purge bleed is activated bypartially opening V6 while V3 is open. When the process gas cylinder isremoved, a pre-set flow of nitrogen will flow from the pigtailpreventing atmospheric intrusion.

Post Purge Routine

Vacuum Mode

With V4 on and V2 opened, a vacuum of 22-24 inches Hg is generatedbetween V1 and the cylinder valve.

Nitrogen Pressure Mode

With V4 on, V2 and V3 opened, V6 is opened allowing nitrogen pressure(80 psi) to overcome the vacuum and to pressurize the system withnitrogen between V1 and the cylinder valve. When V6 is closed, a vacuumis then again generated between V1 and the cylinder valve. V6 is cycledon and off multiple times in the pre-purge to create the vacuum/pressurepurging action. This sequence ends with the pigtail up to V1 undervacuum.

Process Gas Flush

The process gas cylinder valve is now opened. V4 is turned on and thenV1 is opened. Next V5 is cycled on and off allowing process gas to beflushed through the vent through an orifice. Once this sequence iscomplete the system returns to the Process Gas Delivery Mode.

FIG. 13 illustrates schematically the provision a pure PFC stream 120 toa gas cabinet 150 (the internals are not illustrated for clarity). Gascabinet 150 has a vent tube or conduit 180 which leads td a membraneseparator unit 200 having a non-permeate stream 220 and a permeatestream 240 as explained herein.

FIG. 14 illustrates- schematically the provision of multiple (in thiscase three) gas cabinets 150 a, 150 b, and 150 c, all venting into acommon membrane recovery unit 200.

Various examples will now exemplify aspects of the invention.

EXAMPLES Example 1

A feed stream comprising 0.95% vol. C₂F₆, 1.03% vol. CHF₃, 1.10% CF₄,and 96.93% nitrogen at a pressure of 544,218 Pascal, a temperature of293 K (20° C.) and a flowrate of 193 sl/m (standard liter per minute) isfed on the feed side of a polyimide membrane made according to U.S. Pat.No. 5,085,676. A vacuum system creates a low pressure on the other sideof the membrane: the permeate stream recovered is at a pressure of 6,579Pascal, a temperature of 293 K and a flowrate of 181 sl/m, while on thenon-permeate side, the pressure remains 544,218 Pascal, the temperature293 K and the flowrate 12 sl/m. The non-permeate (concentrated) streamfrom the membrane comprises:

C₂F₆ 15.66% vol.

CHF₃ 9.54% vol.

CF₄ 18.15% vol.

N₂ 56.65% vol.

The permeate stream from the membrane comprises:

C₂F₆ 0.02% vol.

CHF₃ 0.48% vol.

CF₄ 0.01% vol.

N₂ 99.49% vol.

The non-permeate stream is further sent to a cryogenic condensationsystem as disclosed hereabove wherein 0.4942 pound of liquid nitrogenper pound of non-permeate stream is contacted by heat exchange, thuscondensing most of the PFCs as indicated hereafter. The compositions ofthe vapor and liquid streams are the following:

Vapor Stream:

C₂F₆ 1.03% vol.

CHF₃ 0.69% vol.

CF₄ 16.5% vol.

N₂ 81.78% vol.

This vapor stream comprises essentially CF₄ diluted in nitrogen.

Liquid Stream

C₂F₆ 47.83% vol.

CHF₃ 29.00% vol.

CF₄ 21.81% vol.

N₂ 1.37% vol.

The liquid stream is essentially concentrated into three liquid speciesC₂F₆, CHF₃ and CF₄. The liquid stream is then either recycled into theprocess from where the feed flow was coming or recovered and shipped forfurther treatment (concentration, separation, etc.)

The vapor stream is preferably recycled to the input of the cryogeniccondensation system or may be treated (for example scrubbed) anddiscarded.

Example 2

Under the same conditions as in Example 1, a feed stream comprising0.95% vol. C₂F₆, 1.03% vol. CHF₃, 1.10% CF₄, and 96.93% nitrogen at apressure of 5,44. 10⁵ Pascal, a temperature of 20° C. and a flowrate of193 sl/m (standard liter per minute) is sent on the same membrane as inExample 1, said membrane being connected to the same cryogenicseparation system using liquid nitrogen. The non-permeate (concentrated)stream from the membrane comprises:

C₂F₆ 15.66% vol.

CHF₃ 9.54% vol.

CF₄ 18.15% vol.

N₂ 56.65% vol.

at the same temperature and pressure as the feed stream, but at aflowrate of 12 sl/m. The permeate stream from the membrane comprises:

C₂F₆ 0.02% vol.

CHF₃ 0.48% vol.

CF₄ 0.01% vol.

N₂ 99.49% vol.

The non-permeate stream is further sent to the cryogenic separationsystem disclosed in Example 1 and the following vapor and liquid streamsare obtained:

Vapor Stream:

C₂F₆ 1.03% vol.

CHF₃ 0.69% vol.

CF₄ 16.5% vol.

N₂ 81.78% vol.

Liquid Stream

C₂F₆ 47.83%

CHF₃ 29.00%

CF₄ 21.81%

N₂ 1.37%.

The liquid stream is essentially concentrated into three liquid speciesC₂F₆, CHF₃ and CF₄. The liquid and vapor streams are e.g. treated asexplained in Example 1.

Example 3

Under the same conditions as in Example 1, a feed stream comprising0.20% vol. C₂F₆, 0.01% vol. CHF₃, 0.06% CF₄, 0.01% NF₃, 0.01% SF₆ and99.71% nitrogen at a pressure of 714,286 Pascal, a temperature of 20° C.and a flowrate of 199 sl/m (standard liter per minute) is sent on thesame membrane as in Example 1, said membrane being connected to the samecryogenic separation system using liquid nitrogen. The non-permeate(concentrated) stream from the membrane comprises:

C₂F₆ 0.5381% vol.

CHF₃ 0.02% vol.

CF₄ 0.1611% vol.

NF₃ 0.0245% vol.

SF₆ 0.0271% vol.

N₂ 99.2291% vol.

(At the same temperature and pressure than the feed stream, but at aflowrate of 73 sl/m) The permeate stream from the membrane comprises:

C₂F₆ 0.0041% vol.

CHF₃ 0.0047% vol.

CF₄ 0.0014% vol.

NF₃ 0.0016% vol.

SF₆ 0.0004% vol.

N₂ 99.9878% vol.

The pressure of the permeate is 6579 Pascal with a flowrate of 126 sl/m.The non-permeate stream is further sent to the cryogenic separationsystem disclosed in Example 1 (0.4335 pound of LN₂ for each pound ofnon-permeate stream) and the following vapor and liquid stream areobtained:

Vapor Stream: C₂F₆ 0.3418% Pressure: 714,286 Pascal CHF₃ 0.0125%Temperature: 144 K. CF₄ 0.1592% Flowrate: 72.8 sl/m. NF₃ 0.0242% SF₆0.0118% N₂ 99.4505%  Liquid Stream C₂F₆ 85.9100%  Pressure: 714,286Pascal CHF₃ 3.2800% Temperature: 144 K. CF₄ 0.9900% Flowrate: 0.2 sl/m.NF₃ 0.1400% SF₆ 6.6900% N₂ 2.9900%

Example 4

Under the same conditions as in Example 1, a feed stream comprising0.20% vol. C₂F₆, 0.01% vol. CHF₃, 0.06% CF₄, 0.01% NF₃, 0.01% SF₆ and99.71% nitrogen at a pressure of 319,728 Pascal, a temperature of 20° C.and a flowrate of 170 sl/m (standard liter per minute) is sent on thesame membrane as in Example 1, said membrane being connected to the samecryogenic separation system using liquid nitrogen. The non-permeate(concentrated) stream from the membrane comprises:

C₂F₆ 0.5600% vol.

CHF₃ 0.0200% vol.

CF₄ 0.1700% vol.

NF₃ 0.0300% vol.

SF₆ 0.0300% vol.

N₂ 99.2000% vol.

(At the same temperature and pressure than the feed stream, but at aflowrate of 112 sl/m.) The permeate stream from the membrane comprises:

C₂F₆ 0.0154% vol.

CHF₃ 0.0041% vol.

CF₄ 0.0039% vol.

NF₃ 0.0019% vol.

SF₆ 0.0009% vol.

N₂ 99.9738% vol.

The pressure of the permeate is 6579 Pascal with a flowrate of 112 sl/m.The non-permeate stream is further sent to the cryogenic separationsystem disclosed in Example 1 (0.4335 lb. of LN₂ for each lb. ofnon-permeate stream) and the following vapor and liquid stream areobtained:

Vapor Stream: C₂F₆ 0.0072% Pressure: 714,286 Pascal CHF₃ 0.0003%Temperature: 144 K. CF₄ 0.1145% Flowrate: 72.8 sl/m. NF₃ 0.0197% SF₆0.0003% N₂ 99.8580%  Liquid Stream C₂F₆ 80.67%   Pressure: 714,286Pascal CHF₃ 2.88%  Temperature: 144 K. CF₄ 8.21%  Flowrate: 0.2 sl/m.NF₃ 1.52%  SF₆ 4.34%  N₂ 2.38% 

Example 5

Under the same conditions as in Example 2, a feed stream comprising1.00% vol. C₂F₆, 0.01% vol. CHF₃, 0.01% CF₄, and 98.96% nitrogen at apressure of 866,595 Pascal, a temperature of 20° C. and a flowrate of5,000 sl/m (standard liter per minute) is sent on the same membrane(first membrane) as in Example 1, said membrane being connected to asecond membrane (cascade connection: non-permeate side of the first tothe feed side of the second). The non-permeate (concentrated) streamfrom the first membrane comprises:

C₂F₆, 33.93% vol.

CHF₃ 0.17% vol.

CF₄ 0.31% vol.

NF₃ 0.17% vol.

SF₆ 0.31% vol.

N₂ 65.11% vol.

At the same temperature and pressure than the feed stream, but at aflowrate of 150 sl/m. The permeate stream from the first membranecomprises:

C₂F₆ 0.0012% vol.

CHF₃ 0.0053% vol.

CF₄ 0.0009% vol.

NF₃ 0.0052% vol.

SF₆ 0.0009% vol.

N₂ 99.9865% vol.

The non-permeate (concentrated) stream from the second membranecomprises:

C₂F₆ 96.52% vol.

CHF₃ 0.23% vol.

CF₄ 0.81% vol.

NF₃ 0.24% vol.

SF₆ 0.81% vol.

N₂ 0.39% vol.

The permeate stream from the second membrane comprises:

C₂F₆ 0.0636% vol.

CHF₃ 0.1358% vol.

CF₄ 0.0424% vol.

NF₃ 0.1339% vol.

SF₆ 0.0406% vol.

N₂ 99.58739% vol.

Example 6

Under the same conditions as in Example 2, a feed stream comprising1.00% vol. C₂F₆, 0.2% CF₄, and 98.9% nitrogen at a pressure of 213,315Pascal, a temperature of 20° C. and a flowrate of 6,366 grams/min, issent on the same membrane as in Example 1, said membrane being connectedto a vacuum switch adsorption system (VSA) with a switching time of 15min The non-permeate (concentrated) stream from the membrane comprises:

C₂F₆ 74.2% wt.

CF₄ 10.8% wt.

N₂ 15.1% wt.

At the same temperature and pressure than the feed stream, but at aflowrate of 84 gams/min.

The permeate stream from the membrane comprises:

C₂F₆ 0.001% wt.

CF₄ 0.014% wt.

N₂ 99.985% wt.

The VSA non-adsorbed stream comprises:

C₂F₆ 94.9% wt.

CF₄ 5.1% wt.

The VSA adsorbed stream comprises:

CF₄ 30.9% wt.

N₂ 69.1% wt.

Example 7

A system of the invention was used to recover PFCs from an effluentstream from a semiconductor tool. A first membrane separation unitincluded three hollow fiber bundles, while a second membrane separationunit included only one hollow fiber bundle. Each hollow fiber bundle wasequal in surface area; thus the first membrane unit provided three timesthe surface area for mass transfer than did the first bundle. Eachbundle also used the hollow fibers described in Example 1. A feed streamcomprising 2083 ppm C₂F₆, 595 ppm CF₄, and balance nitrogen, at apressure of about 540 kiloPascal, a temperature of 305 K (32° C.) and aflowrate of 201 scfh, or 95 sl/m (standard liter per minute) was fed onthe feed side of a polyimide membrane made according to U.S. Pat. No.5,085,676. The PFC material balance indicated that the PFC flow in thefeed was about 0.4590 scf, or about 0.217 sl/m.; the product(non-permeate stream from the second membrane unit) had a PFCconcentration of about 64.7%, and a nitrogen concentration of about35.3%. The PFC recovered in the non-permeate from the second membranewas about 0.457 scfh. or 0.216 sl/m, for a PFC recovery of about 99.5%.

Further modifications to the invention will be apparent to those skilledin the art, and the scope of the following claims are not intended to beunfairly limited by the foregoing description.

What is claimed is:
 1. A process for the separation of perfluorocompoundgases from a gas mixture, said process comprising the step of:contacting a gas mixture comprising at least one carrier gas and atleast one perfluorocompound gas selected from the group consisting ofCF₄, C₂F₆, C₃F₈, C₄F₁₀, and NF₃ with a membrane which permeates said atleast one carrier gas faster than said at least one perfluorocompoundgas.
 2. The process according to claim 1, wherein said at least onecarrier gas is air, nitrogen, oxygen, argon, helium, or mixturesthereof.
 3. The process according to claim 1, wherein said membrane is aglassy polymeric membrane.
 4. The process according to claim 1, whereinsaid at least one perfluorocompound gas is CF₄.
 5. The process accordingto claim 1, wherein said at least one perfluorocompound gas is C₂F₆. 6.The process according to claim 1, wherein said at least oneperfluorocompound gas is C₃F₈.
 7. The process according to claim 1,wherein said at least one perfluorocompound gas is C₄F₁₀.
 8. The processaccording to claim 1, wherein said at least one perfluorocompound gas isNF₃.